What is the difference between...
You have done everything you could do.
... and...
You have done everything you can do.
.. in terms of meaning?
Does "could" refer to the past or to something less sure?
Maybe the first one is incorrect because "could" refers to the past and have done up to now.
Most people will view the sentences as being indistinguishable in meaning.
You did everything you could do
definitely refers to the past, but may imply that the situation persists into the present.
You have done everything you could have done
definitely refers to the past, but emphasizes that it is the recent past and does not deny that the situation persists into the present.
You have done everything you could do
usually refers just to the recent past, but does not deny explicitly that, as of now, you have the ability to do more than you used to.
You have done everything that you can do
usually refers to recent past and present.
Even native speakers, however, are seldom sensitive to the nuances of tense, particularly not in speech or informal writing, and will, if it is important to distinguish among those meanings, do so with with clarifying phrases or clauses rather than relying solely on tense.
Both options are possible in the conversational speech in some known dialects of English. Though, in the modern written speech the sentence You have done everything you can do is rare and historical. You have done everything you could do is more widely used, but, it needs a clause or phrase denoting purpose of the action. For example, You have done everything you could do to fulfil the task.Without such clause, the sentence You have done everything you could do is semantically somewhat incomplete.
